The invention pertains to the art of electrically actuated vent or flue dampers useful for saving energy in furnaces such as those typically used domestically.
Some automatic flue dampers of which I am aware are actuated to their closed position by small synchronous electric motors suitably geared down so as to require 15 to 30 seconds to move the dampers from open to closed positions. Clockwork mechanisms are generally suitable and are therefore almost universally used. To assure the flue damper will always be open when a power failure exists, it is also customary for the operator mechanism to be biased toward the open position by a spring or springs strong enough to overcome the friction in the damper mechanism and in the clockwork actuator. This assures that whenever the driving motor is deenergized, whether by power failure or by normal switching, the damper will be returned to the open position.
It is apparent that a strain is put upon the speed reduction gearing of the mechanism each time that the motor drives the damper into the stop at the damper closed position. Fortunately, standard clockwork gearing is designed to survive this shock for hundreds of thousands of operations. However, the shock is significantly greater when the damper hits the stop in its open position. The reason for this is that since the return springs must be strong enough to overcome the static friction of the clockwork gear train with a reasonable margin of safety, it follows that during the damper opening cycle the speed of movement will be continually accelerated. In a typical mechanism, the speed of the clock motor and gears is 5 to 6 times normal when the damper reaches the full open position. This means that the energy stored in the rotor of the clock motor is 25 to 36 times as great as when it is normally powered on the closing stroke. When the mechanism encounters the stop at its full open position the strain on the gearing is so high that the service life of the device may be quite short; so short in fact that the mechanism may not be able to meet the standards of the regulatory agencies whose approval is required for most installations.
One way of solving the problem is to provide a relatively resilient stop with significant cushioning at the full open position to reduce the strain on the gearing at this point. In my view, this approach is not completely satisfactory since the open damper position needs to be rather well defined and this limits the resilience which can be tolerated.
Of course an obvious solution to the problem is to use nonstandard, extra-strength gearing in the clockwork mechanism. I do not favor this approach since it results in a significant increase in the cost of the mechanism with the attendant disadvantages to all concerned.
It is the aim of my invention to solve the problem by absorbing the stored energy of the clock motor rotor at the end of the damper opening stroke by electrical means without imposing any strain upon the gearing. It is also the aim of my invention that in absorbing the stored energy by the application of electrical means that the basic reliability of the system in the sense of the damper always opening when it is supposed to open is not impaired.